[Trac #475] Touchpads are too "noisy" / Writing input doesn't work

[Zarro Boogs per Child]

#475: Touchpads are too "noisy" / Writing input doesn't work
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 Reporter:  mfoster   |       Owner:  mfoster
     Type:  defect    |      Status:  new    
 Priority:  blocker   |   Milestone:  BTest-2
Component:  hardware  |    Keywords:         
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 The touchpad assembly on the B1 systems was built incorrectly by Alps, the
 manufacturer of the dual-mode touchpad.  An internal ground layer was
 added in contrast to OLPC's guidance, causing insufficient capacitive
 sensing signal amplitude.  As a result, the touchpad is far more noise
 sensitive than it should be.

 The real fix is to remove the internal ground plane layer, which
 dramatically increases the available signal sensing voltage.  In the
 interim, the decision was made to disconnect the resistive sensor and the
 ground plane in the B1 build, which is an effective workaround.  However,
 the proper fix in the B2 systems will further improve touchpad accuracy
 and tracking, and will allow proper operation of both the capacitive and
 resistive sensors.

 One consequence of this change is that elimination of the ground layer
 will have the positive outcome of reducing the force required for written
 input.  As documented in Trac #474, the ESD ramifications of removing this
 ground plane will be avoided by introducing a new metal plate that
 underlies the touchpad sensor assembly.

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Ticket URL: <http://dev.laptop.org/ticket/475>
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